1. Field of Invention
This invention relates generally to ultrasonic liquid level meters of the echo-ranging type that are compensated for environmental changes, and more particularly to a meter of this type which yields both reference and liquid level echo pulses and which includes a time-shared automatic gain control circuit to derive from these echo pulses, output pulses of constant amplitude to facilitate accurate measurement.
2. Status of Prior Art
In an ultrasonic echo-ranging meter, pulses of ultrasonic energy transmitted from a transducer station placed above the surface of a liquid in a tank or open channel are reflected thereby to produce echo pulses which are received at the station. By determining the round trip transit time of the pulse energy in the gaseous medium above the liquid surface, which transit time depends on the distance between the station and the surface, one is able to provide a reading of liquid level.
The accuracy of an ultrasonic liquid level meter of the echo-ranging type is adversely affected by environmental changes; notably temperature, pressure and chemical composition. These factors alter the velocity of acoustic propagation. For example, the velocity of sound in air at 0.degree. C. is 1,087.42 fps, whereas in carbon dioxide it is 1,106 fps. When a meter is installed in an environment in which the chemical nature of the gaseous medium undergoes change, this factor will disturb the level reading unless means are provided to compensate or correct therefor. Similarly, changes in the temperature of the medium or in ambient pressure adversely affects the accuracy of the instrument.
To provide a reading in an echo-ranging liquid level meter that is independent of changes in the propagation medium (air or other gas), Willis et al. U.S. Pat. No. 3,834,233, discloses a first transducer mounted on top of a tank to direct sound energy down into the tank and to detect an echo from the surface of the liquid therein. To compensate for inaccuracies due to changes in the velocity of the sound, Willis et al. positions a second or reference transducer a fixed distance from the first to detect the transmitted wave. Detected signals derived from the two transducers are processed to cancel the effects of any variation in the velocity of sound due to environmental fluctuations.
In my prior U.S. Pat. No. 4,470,299 (Soltz), compensation for environmental changes is effected by a reflector fixedly positioned to intercept and reflect energy from a side portion of the radiation field pattern of the transmitted beam to produce a reference echo signal which in no way interferes with the main liquid level echo signal derived from transmitted energy in a path normal to the surface of the liquid.
In the system disclosed in my prior '299 patent, the transducer is excited to emit periodic pulses which are directed along a center path toward the liquid surface and reflected to produce liquid echo pulses which return to the transducer and are detected thereby. The reference reflector which is placed at a predetermined position relative to the transducer intercepts energy from a side path in the radiation pattern of the transducer to return it to the transducer to produce reference echo pulses. Means are provided to determine the transit time along the center path and along the side path. The ratio of the reference side path and center path transit times is computed to provide an output representing the level of liquid independent of changes in the gaseous environment.
In prior art ultrasonic meters such as those disclosed in the Tankin U.S. Pat. No. 3,090,224, the Kritz U.S. Pat. No. 2,949,772, the Kohno U.S. Pat. No. 4,183,244 and the Asada U.S. Pat. No. 3,710,021, use is made of an automatic gain control circuit in conjunction with the received signals. Automatic gain is generally effected by a control circuit adapted to automatically modify the amplification gain of a receiver in a manner whereby the desired output signal remains at a constant amplitude despite variations in input signal strength.
In an ultrasonic echo-ranging liquid level meter, variations in the amplitude of the echo pulses received from the surface of the liquid are encountered by reason of changes in this surface as well as changes in distance due to liquid level changes. Thus an echo pulse which has a long distance to travel before reaching the transducer will be weaker than an echo pulse traveling a shorter distance.
But in the context of an echo-ranging system of the type disclosed in my prior patent '299 ; in which reference echo pulses as well as liquid level echo pulses are received, at first blush it would appear that no need exists for automatic gain control with respect to the reference echo pulses. Because these pulses are derived from a reflector having a smooth surface placed a fixed distance from the transducer, all reference echo pulses should have the same strength.
However, typical ultrasonic transducers of the same model, though seemingly alike, nevertheless differ somewhat in sensitivity and exhibit a wide spread in echo response. Thus when manufacturing ultrasonic echo-ranging instruments, all of which incorporate the same model of transducer, it becomes necessary to make an individual gain setting to match a particular transducer to the instrument.
Hence in an environmentally-compensated ultrasonic instrument of the type disclosed in m prior '299 patent in which reference as well as liquid level echo pulses are received, actually two automatic gain control functions are needed: one for the reference echo pulses, and the other for the liquid level pulses.
To obviate the need for two automatic gain control circuits in an instrument of the type disclosed in my prior '299 patent, my subsequent U.S. Pat. No. 4,578,997 (Soltz), makes use of a single automatic gain control circuit that is time shared to effect separate gain control for operation in the reference mode and in the liquid level or target mode. In the arrangement disclosed in my '997 patent, the AGC is enabled in a reference mode during a time slot or window having a predetermined duration to effect gain control for the reference echo pulses, and the AGC is thereafter similarly enabled in the target mode to effect gain control for the liquid echo pulses.
We have since found that while the time-shared AGC arrangement disclosed in my prior '997 patent in most cases gives rise to a considerable improvement, there are remaining cases where the reference echo pulse signal shape causes the AGC to favor the reference echo pulse of highest amplitude which is not necessarily the first reference echo pulse. This results in an error in the reference distance count and therefore produces a significant error in level measurements.